The present invention relates to a print head comprising heating resistors as electro-thermal conversion elements, and a print apparatus using the same.
Ink-jet print methods have received a lot of attention owing to their advantageous features, i.e., since they can reduce noise upon printing to a negligible level, allows high-speed printing, can print an image on a so-called normal paper sheet by fixing an ink without requiring any special processing, and so on.
Of these methods, an ink-jet method described in Japanese Patent Publication No. 54-51837 and DOLS (German Laid-Open) No. 2843064 has a feature different from other ink-jet print methods in that heat energy is applied to a liquid to obtain a driving force for ejecting liquid droplets. More specifically, in the print method disclosed in the above-mentioned references, a liquid undergoes a change in state accompanied by an abrupt increase in volume upon application of heat energy, and a liquid is ejected from orifices at the distal end of an ink-jet head by the force based on the change in state, thus forming flying liquid droplets. The liquid droplets become attached to a recording medium to attain printing.
In particular, the ink-jet print method disclosed in DOLS No. 2843064 above can be very effectively applied to a so-called drop-on-demand print method. Furthermore, since a full-line type ink-jet print head having a high-density multi-orifice structure can be easily realized, an image with a high resolution and high image quality can be obtained at high speed.
An ink-jet print head of an apparatus applied to this print method includes a print head board which comprises orifices arranged for ejecting a liquid, liquid ejection portions having liquid channels each including a heat applying portion as a portion for applying heat energy to a liquid for ejecting a liquid droplet, and electro-thermal conversion elements (heating resistors) as means for generating heat energy.
In recent years, as the above-mentioned print head board, one, in which an array of a plurality of heating resistors, drivers which have a one-to-one correspondence with these heating resistors and drive the heating resistors in correspondence with image data, a shift register which has the same number of bits as the heating resistors and parallelly outputs serially input image data to the drivers, and a latch circuit for temporarily storing data output from the shift register are arranged on a single circuit board, has been developed.
FIG. 12 shows the circuit arrangement of such a conventional print head board 300. Referring to FIG. 12, reference numeral 301 denotes an array of heating resistors; 302, power transistors serving as drivers; 303, a latch circuit; and 304, a shift register. Reference numeral 305 denotes a clock signal which is used for shift-inputting data in the shift register 304. Reference numeral 306 denotes serial image data input to the shift register 304. Reference numeral 307 denotes a latch signal; and 308, a heat pulse signal for externally controlling the ON times of the power transistors 302. Reference numeral 309 denotes a logic power supply; and 310, ground. Reference numeral 311 denotes a power supply (VH) input for driving the heating resistors 301.
In a printer apparatus having the head including the print head board with the above-mentioned arrangement, the serial data 306 is serially input to the shift register 304. The image data set in the shift register 304 is latched by the latch circuit 303 in response to the latch signal 307. When the heat pulse signal 308 is input, power transistors 302 corresponding to data "1" of the image data are set in the ON state. In this manner, the corresponding heating resistors 301 are energized and driven, ink in the liquid channels of the driven heating resistors 301 is heated, and the ink drops are ejected from the orifices, thus achieving printing.
Upon consideration of energy required for forming bubbles in a liquid portion contacting the heating resistor 301, if a heat dissipation condition remains the same, the energy corresponds to the product of required input energy per unit area of the heating resistor 301 and the area of the heating resistor 301. For this reason, the voltage applied across both ends of the heating resistor 301, and the current and time (pulse width) flowing through the heating resistor 301 can be set to obtain the above-mentioned energy. In practical use, the voltage can be set to be almost constant by the power supply of the printer apparatus main body. However, as for the current, the resistances of the heating resistors 301 have different values depending on lots and boards due to a variation in film thickness of the heating resistor 301 in the manufacture of the board. Therefore, when the application pulse width is constant, and the resistance of the heating resistor 301 becomes higher than a setting value, the current value decreases, and the application energy becomes insufficient. As a result, the ink cannot form bubbles. On the contrary, when the resistance of the heating resistor 301 becomes small, and the current value flowing through the heating resistor becomes larger than the setting value, excessive energy is input, resulting in burning and short service life of the heating resistor 301. In order to prevent this problem, a sensor 314 always monitors the resistance value of the heating resistor 301, and the power supply voltage or the application pulse width is changed based on the detected resistance value, so as to apply constant energy.
Next, upon consideration of the ejection amount of a liquid droplet to be ejected, the ejection amount is associated with the bubble formation volume of an ink. Since the bubble formation volume of the ink changes depending on the temperature of the heating resistor 301 and the ambient temperature, a pulse (pre-heat pulse) having energy low enough not to eject an ink is applied before an applying of a heat pulse for ejection, so as to adjust the temperature of the heating resistors 301 by the pulse width and timing of the pre-heat pulse. In this manner, a liquid droplet of a predetermined amount is ejected, and a desired print quality is maintained.
According to the above-mentioned prior art, correction of a variation in resistance value of each heating resistor 301 and temperature control of the board can be realized by changing the width of the heat pulse, and the width and timing of the pre-heat pulse to be applied to the heating resistors 301 is changed under the control of the printer apparatus main body by feeding back signals from the sensor 314 for monitoring the resistance value and a temperature sensor 315 for monitoring the temperature, and for outputting the heat signal. However, the ink ejection amounts vary depending on nozzles due to a variation in area of orifice apertures, a variation in thickness of the protection films of the heating resistors 301, and the like in the manufacture in addition to the above-mentioned factors even when the same energy is applied to the heating registers 301. Such variations results in density nonuniformity, stripes, and the like on printed matter, and hence ejection amount control for each nozzle or several nozzles is required.
When a plurality of print head boards are connected in series with each other to form a multi-nozzle ink-jet head to meet demand for an increase in the number of nozzles of an ink-jet head, since each of the print head boards has the heating resistors 301 having different resistance value from those of other print head board, the heat pulse width for ejecting an ink must be changed in each board to generate almost the same energy in the respective boards. As described above, when the print head is constituted by a plurality of boards, the print density difference between adjacent boards becomes conspicuous in addition to the above-mentioned orifice area. For this reason, it becomes more important to correct the ejection amount of ink in units of nozzles (heating resistors) in the board than in the case of a print head constituted by a single board.